Many trailer vehicles have air suspension systems which support the vehicle on one or more wheel bearing axles. Examples of such systems are disclosed in the U.S. Pat. Nos. 3,406,983 to Masser issued Oct. 22, 1968, 3,332,701 to Masser issued July 25, 1967 and 3,547,215 to Bird issued Dec. 15, 1970.
Air supply and control systems are provided to inflate the air springs and to maintain the height of the trailer constant with respect to the axles for many different loads. To this end, height control valves are provided to adjust the flow of air to the air springs. When the trailer is being loaded or unloaded, the trailer is usually parked and the compressor is turned off. Loading and unloading usually results in fluctuation of the load due to a lift truck moving into and out of the trailer, thus repeatedly adjusting the air to the air springs. After a certain number of cycles, the air in the surge tank is too low to operate the springs and the air springs at least partially collapse. Due to the fact that the trailer pivot point is usually at the front of the trailer, vertically above the axles and above the pivot point of the suspensions, the back part of the trailer swings downwardly and forwardly as the air springs collapse thereby moving the back part of the trailer away from the loading dock. In addition to providing a gap between the loading dock and the trailer back portion, the movement of the trailer may but additional stress on the lading gears when the tractor is disconnected from the trailer.
Air spring control systems have been heretofore provided with manually operated valves for dumping the air in the air springs. Thus, a driver may exhaust the air from the air springs before backing the trailer to the loading dock. Alternatively, the operator may keep the truck running during loading and unloading. This latter operation can be fairly expensive and may not be particularly convenient for the driver. Typically, the trailers may sit at the loading dock for days before the loading is complete. Whereas the problem described above may be avoided by the manual operation of the dumping valve, it depends on the operator remembering to operate the valve prior to backing to the loading dock and to reactivate the air system when the loading is complete. The operator may not remember to dump the air at an appropriate time or reinflate the air springs at the proper time. Further, operator controlled dumping of air may be illegal in some states.